1. Field of the Invention
The present invention relates to a handoff method for a mobile station in a wireless local area network (LAN), and more particularly, to a handoff method that secures a quality of service (QoS) in a wireless LAN, and an access point (AP) and a mobile station that perform the handoff method in the wireless LAN.
2. Description of the Related Art
In response to increasing popularity of multimedia networking, researches are being conducted to secure a quality of service (QoS) in a multimedia network. With recognition of wireless networking, markets now demand that a wireless network provide the same QoS of existing wired networks. Standards have been developed with regards to the above issue. In particular, 802.11a and 802.11g focus on providing support for an increased bandwidth, while 802.11e is intended to address the issues of a QoS in a wireless LAN.
However, the development of MAC techniques that support a QoS has been viewed as of little value in a mobile environment. That is, MAC techniques do not support the fast handoff service that is required for a QoS in the mobile environment. Therefore, a QoS for real-time multimedia data cannot be secured due to the delay generated as a mobile station moves from one cell to another cell in a wireless LAN.
FIG. 1 illustrates a roaming process of a mobile station 140 to a subnet Y 120, while a QoS connection is established between a mobile station 150 in a subnet Z 130 and the mobile station 140 in a subnet X 110.
A conventional handoff method in a wireless LAN will be described, that is, moving the mobile station 140 from the subnet X 110 to the subnet Y 120 in a MAC environment. It is assumed that substantial roaming occurs within one extended service set (ESS), and the subnet X 110 and the subnet Y 120 of FIG. 1 are present within one ESS while having portions that overlap each other.
Where the mobile station 140 in the subnet X 110 approaches the subnet Y 120, a channel search operation is performed, due to a weakened signal from the subnet X 110. Where the number of channels in the subnet Y 120 is 12, the mobile station 140 that performs the roaming receives beacon frames from all 12 channels in the subnet Y 120 while in a passive mode, and sends probe request messages to all 12 channels in the subnet Y 120 while in an active mode.
In this scenario, the mobile station 140 selects an access point (AP) from the subnet Y 120 that sends a stronger beacon frame or stronger probe response message. The mobile station 140 sends a reassociation request message to the new AP of subnet Y 120 including information on its previous AP of subnet X 120. The previous AP is unaware of the handoff until the new AP informs the previous AP using an inter access point protocol (IAPP). The APs communicate over a wired LAN or another media.
The conventional roaming service in a wireless LAN, as shown and described with FIG. 1, provides a connectionless-based roaming. Where the mobile station 140 is receiving real-time streaming data, a QoS cannot be guaranteed since the previous AP of subnet X 110 is unaware of the handoff state of the mobile station 140 until the roaming is completed. In addition, the AP of subnet X 110 continuously sends QoS data to the mobile station 140 in the handoff state since the AP does not receive acknowledgement messages with respect to the data. Accordingly, the performance of the subnet X 110 is lowered. In addition, the mobile station 140 may completely lose the QoS stream, and even where the mobile station 140 recovers the QoS stream by using an IAPP, a significant data loss is inevitable.
Another problem of the conventional roaming service is that about 90% to 95% of delays are probe delays of 200 to 400 ms, due to the search of media, as shown in FIG. 2. For example, where 12 channels are present in one subnet, all channels are scanned regardless of channel use. Accordingly, the mobile station 140 sends probe request messages to all channels and waits for the responses according to a MinChannelTime. In addition, even where the mobile station 140 receives a response message, the mobile station 140 waits for a MaxChannelTime to determine whether another AP is using the channel. However, a basic service set (BSS) uses only about four channels to avoid channel interferences. Therefore, the process of scanning of all channels causes unnecessary probe delays.
The handoff method is one of the important fields of a wireless LAN environment. This is because as the speed of the wireless LAN environment increases, frequency bands increase and areas to send data decrease. In addition, due to the demand of a QoS for multimedia data, a faster handoff technology is needed. Even with the enhanced mobile functions with respect to a layer 3 by using an IPv6 technology, the needed handoff technology is not provided because a fast handoff technology is not realized at a MAC layer, i.e., a layer 2.